Fast repeats/flashes video clip
Codecs.

Prefaced with my usual “very difficult to diagnose from a distance.”

All codecs are not created equal, and are complex beasts when it comes to decoding. This is especially true for encoded files that could be subject to massive frame compression such as screen captures.

Best test is to render each source file long sequence into a  stills or AVI JPG format from within Blender prior to editing, and then edit the resultant output.

If this works, and if you are so inclined to learn about the dark alchemy of codecs, I could explain further if required.